This invention relates generally to color printing systems and, more particularly, to color printing systems and methods which address the problem of the bleeding of black ink into adjacent color areas of a page.
The use of computers has proliferated to such an extent that computers are readily available to people from all walks of life. It is not uncommon to enter an office and see a personal computer system on virtually every desk. As illustrated in FIG. 1, the typical computer system 8 has a computer housing 10, a keyboard 12 for user interface with the computer, a color display 14, and (optionally) a printer 16. The printer 16 is sometimes shared with other computers over a local area network (LAN). The computer housing 10 typically contains a central processing unit (CPU), read only memory (ROM), random access memory (RAM), long term memory (such as a hard disk or a PCMCIA card), and other logic, peripherals, interfaces, and drivers well known to those skilled in the art of computer design.
The computer system can store image information in its permanent (e.g. hard disk) or temporary (e.g. RAM) memory. Logically, there is very little difference between permanent and temporary memory during the operation of the computer and, therefore, all forms of read/write memory accessible by the CPU of the computer will be collectively referred to herein as "memory." Since many color displays are RGB (red, green, blue) type monitors, image information is typically stored in memory in the form of an electronic document containing an RGB pixel map and corresponding information regarding the nature of "objects" in the document. As is well known to those skilled in the art, "objects" are collections of code and data which represent a logical entity, such as a text object, a graphic object, or a picture object. In the RGB model or "space", the three colors are additive such that the more of each color that is added, the closer the color in the image is to white. This corresponds to the way color displays work: saturating the red, green, and blue phosphors of a color display produces white, turning the three phosphors completely off produces black, and mixing the three colors produces other colors.
For cost and other reasons, the printer 16 is often a monochrome printer which is capable of printing in only one color. Since the "color" is usually black and since most paper is white, monochrome printers are often called "black-and-white" printers. The computer provides the printer with image information to cause the printer to print an image on a printed paper 18. This image can be represented in the electronic document by objects corresponding to the graphic object OG, the text objects OT, and the picture object OP printed on the page.
Color printers are also available, although at a higher cost (and sometimes lower resolution) than monochrome printers using the same technology. Most color printers are "CMYK" type printers which use utilize cyan (C), magenta (M), yellow (Y), and black (K) to produce color images. In other words, most color printers use a CMYK model and therefore operate in CMYK "space." With CMYK printers, the colors CMY are subtractive. That is, the more of cyan, magenta, or yellow added, the closer the color in the image is to black.
With CMYK printers, black can be theoretically achieved by mixing full density cyan, magenta, and yellow. However, the C, M, and Y inks that are used cannot produce a truly dark, dense black, so a separate black ink is also provided. Since the display operates on in RGB space, the computer converts the RGB pixel map of the electronic document into a CMYK pixel map to create a printed image. However, in the prior art, when the CMYK pixel map is created from the RGB pixel map, the information regarding the nature of objects in the image is not maintained. Therefore, it is not possible to distinguish various objects in a CMYK encoded image.
Due to material limitations, it is difficult to create a black ink which both has the desired printing qualities (darkness, drying time, viscosity, etc.) and yet does not migrate or "wick" into adjacent areas on a page. As illustrated in FIG. 2a, a color image 20 on a page 21 may include a black area 22 and a color area 24. The ink of black area 22 wicks along the fiber in the paper and creates a number of "bleed lines" 26, which are often quite visible in the lighter color area 24 portion of the printout 20. The result of this effect is referred to as "bleed." Excessive bleeding will degrade the image 20.
Since the amount of bleed is directly related to the capillary action or "wicking" of the fibers in the paper, a partial solution to this problem is to use a coated paper to reduce wicking. However, this solution requires the purchase and storage of expensive and sometimes scarce coated paper, as opposed to relatively cheap and readily available standard papers. Furthermore, since different printers may have different ink chemistries, it is often necessary to buy coated paper that is specifically made for that printer. This, of course, again tends to increase cost and reduce availability.
One attempt to avoid bleed without using a special, coated paper is illustrated in FIG. 2b. In this solution the computer system 8 analyzes the CMYK pixel map and alters it by placing a band of white pixels 30 (i.e. where no color is present) between black pixels 32 and the color pixels 34. This reduces the amount of black that bleeds into an adjacent color by providing a buffer zone between the two areas. However, this solution creates a white "halo" surrounding the color areas, which noticeably degrades the quality of the image.
Another attempted solution is illustrated in FIG. 2c. Here the computer system analyzes the CMYK pixel map and alters it by placing a mixed cyan, magenta, and yellow band of pixels 36 at the intersection of black pixels 32' and other color pixels 34'. This reduces black bleed but has the image corrupting side-effect of visibly distorting the color of the perimeter of the color area.
FIG. 2d illustrates yet another partial solution to the bleeding problem. In this solution, the computer system analyzes the CMYK pixel map and alters the map by placing a reduced pixel density black band 38 between the black pixels 32" and other color pixels 34". This reduces black bleed but has the image corrupting side effect of creating a visible band of gray adjacent to the color area.
Yet another approach of the prior art is to replace all the black ink on the page with either reduced pixel density black or with a CMY pixel mix which, as explained previously, can produce a less dense black. While this creates an image without apparent halos or bands, the lack of true black, especially in any text on the image, is noticeable and reduces the contrast and resolution of the image.